WasmAppHost does not serve correct CORS headers for dotnet.native.worker.mjs

Description

I'm trying to do some experiments using the threads support in webassembly projects. With a basic project and minimal configuration it seems to be impossible to start in debug the project because the WasmAppHost tools returns incorrect headers while navigating the app. Looking at the source code I noted that this middleware does not correctly handle files with extension .mjs .

Reproduction Steps

create a new browser wasm project dotnet new wasmbrowser
add <WasmEnableThreads>true</WasmEnableThreads> in the generated .csproj file under PropertyGroup
add a <PackageReference Include="Microsoft.NET.WebAssembly.Threading" Version="9.0.0-rc.1.24431.7" /> under ItemGroup
execute dotnet run or F5 in VS

Expected behavior

The http response for https://127.0.0.1:51986/_framework/dotnet.native.worker.14oy9i9plz.mjs must contains at least the following CORS header:

Cross-Origin-Embedder-Policy:require-corp

Edge report the error as in following screen:

A correct response is like this one for https://127.0.0.1:51986/_framework/dotnet.runtime.xw3lbfxs0o.js

Actual behavior

Currently no CORS headers are available in the response for https://127.0.0.1:51986/_framework/dotnet.native.worker.14oy9i9plz.mjs and so application bootstrap is interrupted by the browser.

Regression?

Not tested in previous versions

Known Workarounds

Configuration

.NET SDK:
 Version:           9.0.100-rc.1.24452.12
 Commit:            81a714c6d3
 Workload version:  9.0.100-manifests.977c08b4
 MSBuild version:   17.12.0-preview-24422-09+d17ec720d

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.22631
 OS Platform: Windows
 RID:         win-x64
 Base Path:   C:\Program Files\dotnet\sdk\9.0.100-rc.1.24452.12\

.NET workloads installed:
Configured to use loose manifests when installing new manifests.
 [android]
   Installation Source: SDK 9.0.100-rc.1, VS 17.11.35312.102, VS 17.12.35323.107
   Manifest Version:    35.0.0-rc.1.80/9.0.100-rc.1
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100-rc.1\microsoft.net.sdk.android\35.0.0-rc.1.80\WorkloadManifest.json
   Install Type:              Msi

 [aspire]
   Installation Source: SDK 9.0.100-rc.1, VS 17.11.35312.102, VS 17.12.35323.107
   Manifest Version:    8.2.1/8.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\8.0.100\microsoft.net.sdk.aspire\8.2.1\WorkloadManifest.json
   Install Type:        FileBased

 [ios]
   Installation Source: SDK 9.0.100-rc.1, VS 17.11.35312.102, VS 17.12.35323.107
   Manifest Version:    17.5.9270-net9-rc1/9.0.100-rc.1
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100-rc.1\microsoft.net.sdk.ios\17.5.9270-net9-rc1\WorkloadManifest.json
   Install Type:              Msi

 [maccatalyst]
   Installation Source: SDK 9.0.100-rc.1, VS 17.11.35312.102, VS 17.12.35323.107
   Manifest Version:    17.5.9270-net9-rc1/9.0.100-rc.1
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100-rc.1\microsoft.net.sdk.maccatalyst\17.5.9270-net9-rc1\WorkloadManifest.json
   Install Type:              Msi

 [maui-windows]
   Installation Source: SDK 9.0.100-rc.1, VS 17.11.35312.102, VS 17.12.35323.107
   Manifest Version:    9.0.0-rc.1.24453.9/9.0.100-rc.1
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100-rc.1\microsoft.net.sdk.maui\9.0.0-rc.1.24453.9\WorkloadManifest.json
   Install Type:              Msi

 [wasm-experimental]
   Installation Source: SDK 9.0.100-rc.1
   Manifest Version:    9.0.0-rc.1.24431.7/9.0.100-rc.1
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100-rc.1\microsoft.net.workload.mono.toolchain.current\9.0.0-rc.1.24431.7\WorkloadManifest.json
   Install Type:              Msi

 [wasm-tools]
   Installation Source: SDK 9.0.100-rc.1, VS 17.11.35312.102, VS 17.12.35323.107
   Manifest Version:    9.0.0-rc.1.24431.7/9.0.100-rc.1
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100-rc.1\microsoft.net.workload.mono.toolchain.current\9.0.0-rc.1.24431.7\WorkloadManifest.json
   Install Type:              Msi

Host:
  Version:      9.0.0-rc.1.24431.7
  Architecture: x64
  Commit:       static

.NET SDKs installed:
  8.0.400 [C:\Program Files\dotnet\sdk]
  9.0.100-rc.1.24452.12 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 6.0.33 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 8.0.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.0-rc.1.24452.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.33 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 8.0.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.0-rc.1.24431.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 6.0.33 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 8.0.8 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 9.0.0-rc.1.24452.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other information

No response

dotnet / runtime